"A good stock of examples, as large as possible, is indispensable for a thorough understanding of any concept, and when I want to learn something new, I make it my first job to build one." – Paul Halmos

Out of curiosity, what’s your main source of learning/knowledge if not books? I’m guessing that right now you’re probably at the point where most of your knowledge comes from attending lectures and conferences, talking to other mathematicians, and reading papers. But I’m more interested in how you got to this point, especially during your undergrad years, since your pace of learning would be impossible to maintain solely on undergrad math courses. If not primarily books, what sources did you use (and would recommend) to quickly build up a solid foundation of mathematical knowledge pre-grad school?